Safety device for die casting machines

ABSTRACT

A SAFETY DEVICE IN A DIE CASTING MACHINE HAS A MOLDHOLDING PRESS CONSTRUCTED AS A TOGGLE JOINT PRESS WITH A HYDRAULIC DRIVE, AND IN WHICH THE FEEDING OF THE CASTING MATERIAL FROM THE PRESSURE CHAMBER INTO THE MOLD CAVITY IS INITIATED BY AN ELECTRIC SWITCH ELEMENT. IN THE CONTROL CIRCUIT OF THE SWITCH ELEMENT VARIOUS SWITCHES, RESPONDING TO THE POSITION OF THE HYDRAULIC DRIVE OR TO A MINIMUM PRESURE PREVAILING THEREIN, ARE ARRANGED IN SERIES IN SUCH A WAY THAT THE SWITCH ELEMENT THAT INITIATES THE CASTING PROCESS RESPONS ONLY WHEN THE DRIVE SYSTEM HAS ASSUMED ITS POSITION OF CLOSURE AND THE TWO MOLD HALVES ARE FIRMLY PRESSED AGAINST EACH OTHER.

Nov. 7, 1972 R. FREY SAFETY DEVICE FOR DIE CASTING MACHINES Filed July 6, 1971 United States Patent 3,702,153 SAFETY DEVICE FOR DIE CASTING MACHINES Rolf Frey, Winterbach, Germany, assignor to Mahle- Werk G.m.b.H., Fellbach, Germany Filed July 6, 1971, Ser. No. 159,995 Int. Cl. B22c 19/04; B22d 17/32 US. Cl. 164-153 1 Claim ABSTRACT OF THE DISCLOSURE A safety device in a die casting machine has a moldholding press constructed as a toggle joint press with a hydraulic drive, and in which the feeding of the casting material from the pressure chamber into the mold cavity is initiated by an electric switch element. In the control circuit of the switch element various switches, responding to the position of the hydraulic drive or to a minimum pressure prevailing therein, are arranged in series in such a way that the switch element that initiates the casting process responds only when the drive system has assumed its position of closure and the two mold halves are firmly pressed against each other.

This invention relates to a safety device for die casting machines whose mold-holding press is constructed as toggle joint press with a hydraulic drive, and in which the feeding of the casting material from the pressure chamber into the mold cavity is initiated by an electric switch element in whose control circuit a switch is inserted which responds when that position of the hydraulic drive is reached which corresponds to the position of complete closure of the mold-holding press.

The purpose of such safety device consists in delaying the beginning of the mold-filling process until the mold is completely closed and the closing pressure acting thereon is sufficiently high. However, if the safety device consists exclusively of the aforementioned switch, this structure is not sufficient for achieving in all cases the objects sought. The switch, to be sure, makes it possible to make the initiation of the molding process a function of the arrival of the hydraulic drive at that position of operation at which the mold-holding press is normally completely closed. However, it is definitely possible, that, as a result of minor deformations in elongation, e.g., due to tempera:

ture changes, of the rod which connects the stationary mold-supporting plate with the crosshead of the press, or as a result of faulty adjustment, the two mold halves do not even touch each other when the hydraulic drive has reached the position in which the press should actually be completely closed, but in fact, is not closed yet although the toggle mechanism has already reached its extended terminal position. Should in this case initiation of the casting process be permitted, the liquid casting material would escape from the gap between the two mold halves. Likewise, in the case of faulty adjustment, the mold might be closed while a sufiicient closing pressure might be lacking.

The object of this invention is to avoid such drawbacks and to construct a device of the initially mentioned kind such that it offers with the aid of a simple full protection against an initiation .of the casting process while the mold is not yet closed or not closed with sufiicient force.

In general these objects are obtained by including in the safety device means responding to the closing pressure. This can be simply accomplished in die casting machines with purely hydraulically driven mold-holding presses since in this case the pressure in the hydraulic drive system corresponds directly to the closing pressure. However, this solution of the aforementioned problem meets with considerable difiiculties in die casting machines havice ing only a control switch responsive to the mold closed position since in such machines the closing pressure is not proportional to the pressure in the hydraulic drive system and can be determined directly only by measuring the tensile stress prevailing in the guide rods of the moldholding press, which procedure, however, is disproportionately expensive and uneconomical.

These difficulties are now overcome by this invention on the basis of the fact that the pressure in the hydraulic drive system of the toggle press of a die casting machine is subject to characteristic variations during the closing movement on account of the change in the transmission ratio which varies during the said closing movement. First a pressure maximum occurs during the start. After the starting resistance has been overcome, the pressure drops slowly and then increases slowly to a maximum which occurs shortly before the extended terminal position of the toggle mechanism has been reached or shortly after the moment at which the two mold halves, when the adjustment is correct, just touch each other. In the further course of the closing movement the pressure again drops noticeably, although a greater closing force must be produced at this time since the transmission ratio of the toggle mechanism approaches an infinite value. Another pressure increase, of no interest in this connection, takes place only when a limit stop has been reached which marks the terminal position. In this process, a relatively long stroke of the closure piston of the hydraulic drive, which can be utilized for initiating switch movements, corresponds to the short period between the time of maximum pressures and that of the extended terminal position of the toggle mechanism.

This utilization, according to this invention, takes place in a device of the initially mentioned kind, by an insertion in series of a further switch into the circuit of the aforementioned switch which marks the required terminal position of the hydraulic drive. The said further switch is controlled by a self-holding relay in whose circuit two additional series connected switches are inserted; namely, a second switch dependent on the position of the hydraulic drive and responding when the hydraulic drive arrives at a position corresponding to that position of the mold-holding press at which the two mold halves just touch each other, and a switch dependent on the pressure in the hydraulic system, responding only when a prespecified minimum pressure is reached, which corresponds to the pressure maximum that occurs normally shortly before the terminal position is reached.

The means by which the objects of this invention are obtained are described more fully with reference to the accompanying drawings in which:

FIG. 1 is a front view, partially in cross-section, of the toggle joint hydraulically driven mold-holding press showing the switch elements of the saftey device;

FIG. 2 is a circuit diagram; and

FIG. 3 is a pressure P--time t graph of the hydraulic drive of the mold-holding press shown in FIG. 1.

In FIG. 1 is shown the mold-holding press 10 of the die cast machine. It includes a machine frame 'with a stationary mold-clamping plate 11, a press crosshead 13 connected with said mold-clamping plate 11 by guide rods 12, a movable mold-clamping plate 15 hinged to said press crosshead 13 by means of a toggle 14, and a hydraulic drive for the toggle system, which drive consists of a hydraulic cylinder 16 and a piston 17 slidable in said hydraulic cylinder 16, with which piston 17 a piston rod 18 is rigidly connected, which is provided at its end with a switch cam 19.

The two mold-clamping plates 11, 15 support the two mold halves 21, 22, respectively. A pressure chamber 23 ends in the first-mentioned mold half, in which pressure chamber 23 a hydraulically driven pressure piston 24 slides. A corresponding hydraulic device 35 is controlled by a valve device 31, which in turn is controlled by a switch magnet 30 namely, in such a way that, when the switch magnet is energized, valve device 31 is displaced from the position shown in the drawing to the right.

Two switching devices 40 and 50 are coordinated with press hydraulic drive 16, 17, in such a way that a switch 41, FIG. 2, which is open in the position of rest and belongs to switch 40, isclosed by switch cam 19 as soon as they mold-holding press has approached its position of closure to such a degree that normally in the hydraulical drive the above-mentioned pressure maximum, located in FIG. 3 on the line 4, has occurred. Switch device 50 is actuated when the hydraulic drive 16, 17 has reached the position in which the toggle mechanism of the moldholding press is normally completely extended, on the line in FIG. 3.,A switch 51 closed in the position of rest and a switch 52 open in the position of rest, are a part of switch device 50, In addition to these two switch devices, a further switch device 60 is provided which closes a switch 61 as soon as the pressure in the closing cylinder 16v reaches a value which corresponds to the maximum at line 4. A further switch device 70 responding to the position of movable mold supporting plate 15 is arranged in such a way that a switch 71 belonging thereto is open when the mold is fully open, and otherwise closed.

As shown in FIG. 2, switches 41, 51 and 61 are in series with a switching relay 80 which controls two switches 81. and 82. Switch 81 is located, together with switch 71 of switch device 70 in a locking circuit through which the voltage is maintained in relay 80 as long as switch 71 remains closed. Switch 82 is positioned in a circuit of its own in series with switch 52 and a relay switch 90. A switch 91 controlled by the relay switch 90 is positioned in the circuit of magnetic switch 30.

When all parts of the die casting machine are properly adjusted, the device operates as follows:

As soon as the mold-holding press has reached the position at which the two mold halves 21 and 22 touch each other, switch device 40 is actuated by cam 19 and switch 41 is closed. At the same time switch 61 also closes because the pressure in closing cylinder 16 is suflicient to actuate pressure switch device 60. As a result thereof, relay 80 is imparted a voltage and closes the two switches 81 and 82.

Through thealready previously closed switch 71 and through switch 81, relay 80 remains subject to voltage even when switches 41 and 61 drop again since the switch cam has moved on in the meantime and then actuates switch device 50, whereby switch 52 is closed and relay switch 90 is energized, and in turn closes switch 91 and thus also the circuit of magnetic switch 30, whereby the casting process is initiated.

When now in deviation of this normal operation one of the aforementioned defects occurs, the safety device described prevents actuation of relay 90 and thus the initiation of the casting process. When, for instance, at the actuation of switch device 40 the two mold halves 21 and 22 do not yet bear against each other, the lacking resistance prevents formation of the pressure maximum shown in FIG. 3 at line4, and, therefore, switch device does not respond. Therefore, switch can not attract and the circuit of relay remains open, even when through switch device 50 switch 52 is closed.

Having now described the means by which the objects of this invention are obtained I claim:

1. A safety device in a die casting machine comprising mold-holding press constructed asv a toggle joint press with a hydraulic drive and in which the feeding of the casting material from the pressure chamber into the mold cavity is initiated by electrically controlled first switch element means responding when the hydraulic drive reaches the position that corresponds to the position of closure of the mold-holding-press the improvement in which said switch (52) is connected in series with second switch means (82) controlled by a holding relay (80) in whose circuit two additional switches are connected in series comprising a second switch (41) which depends on the position of the hydraulic drive and which responds when the position of the hydraulic drive is reached which corresponds to that position of the mold-holding press in which the two mold halves just touch each other, and a switch (61) dependent on the pressure in the hydraulic system, which switch responds only when a pre-specified minimum pressure is reached.

References Cited UNITED STATES PATENTS 3,029,483 4/ 1962 Mills et a1. 164152 3,310,841 3/1967 Hehl 164153 X 3,534,442 10/1970 Mahle 425 2,433,132 12/1947 Lester 425-149 I. SPENCER OVERHOLSER, Primary Examiner J. E. ROETHEL Assistant Examiner U.S. Cl. X.R. 

